1. Field of the Invention
The invention relates to a process for manufacturing a semiconductor device, and more particularly, to a process for manufacturing a semiconductor device comprising connecting an underlying conductive layer and an overlaying conductive layer through via-holes prepared in an interlayer insulating film, or filling conductive plugs into via-holes formed in an interlayer insulating film.
2. Description of the Related Art
In semiconductor integrated circuits, a certain degree of freedom is given to bonding between elements by a multilayer interconnection, and when establishing continuity between upper and lower conductive layers, the following methods are generally used:
First, a method is used in which via-holes are formed in an interlayer insulating film covering an underlying conductive layer, a metal material such as aluminum is then deposited inside the via-holes and on the interlayer insulating film by an evaporation or sputtering process, and then the metal film is patterned to form an overlaying conductive layer. The formed overlaying layer is generally recessed at the locations of the via-holes.
Second, a method has been proposed in which a conductive plug is filled into a via-hole in an interlayer insulating film, and an overlaying conductive layer in contact with the conductive plug is formed on the interlayer insulating film. According to this method, the conductive layer located on the via-holes can be planarized to thereby locate different via-holes thereon, and thus a degree of freedom of the wiring design can be further increased.
With a high integration of semiconductor devices, the sizes of via-holes have been reduced and aspect ratios increased. Accordingly, when the metal film used for the overlaying conductive layer and the conductive plug is formed by a sputtering or evaporation process, it is hard to deposit the metal material on the inside wall of the via-holes, due to the shadow effect, and thus the metal film is discontinuous within the holes. Consequently, the step coverage may be unsatisfactory, whereby the contact resistance between the conductive plug and the underlying conductive layer is increased, and in the worst case, disconnections may occur in the via-holes. The via-hole size able to be successfully filled by such a prior method, with a metal, is as large as 1 micrometer for an aluminum material
To solve these problems, a method has been proposed in which the metal film is deposited and is formed in the via-holes in the continuous state, by a CVD process free of the shadow effect, to thus improve the coverage, or a method in which the metal film formed on the insulating film and the inside wall of the via-holes is melted by an irradiation by a pulse laser, and the molten metal film is transported as a mass into the via-holes. (The latter method is described in JP-A-37634/63, laid-open on Feb. 18, 1988.)
According to the former method, however, since an organometallic compound gas is generally used for a metal film deposition of, for example, aluminum, problems arise such as where carbons contained in the depositing gas, is included in the metal film, the resistivity of the upper conductive layer and the conductive plug increases. Therefore, the method of using a chemical vapor deposition (CVD) process for the deposition of metal material is now little utilized.
In the latter method, in a state in which the metal film becomes discontinuous in the via-holes, because the molten metal film has a low wetting property to the interlayer insulating film, disadvantages arise such as a mass transportation of the metal film melted by the pulse laser irradiation to the top or bottom of the via-holes, producing voids therein
JP-A-61146/59, laid-open on Apr. 7, 1984, discloses a method comprising forming a first conductive film on an insulating film and portions whereat contact holes are preformed, the conductive film consisting of a metal or metal silicide having a high melting point formed by a vapor deposition process, coating the first conductive film with a second conductive film consisting of a metal having a low melting point, and melting the second conductor film.
In the method disclosed in JP-A-61146/59, the second conductive film having a lower melting point is only fused, and the first conductor film having a higher melting point is left in a solid state. Further, the melting of the second film with a lower melting point is effected by merely heating the second film together with both of the other layers or films and the substrate, for a very long time, so that the entire article to be processed is brought to an elevated temperature at which the second film is fully melted.
In the prior art mentioned above, the difficulty of making a second film continuous is not considered. (The prior art is based on the premise that a sputter process gives a continuous film.) To obtain a fully filled conductive plug in a small via-hole, a film to be melted must be continuous, as mentioned above.